The invention relates generally to analog to digital converters (ADC""s), and more particularly to multistage analog to digital converters.
Analog to digital converters (ADC""s) are used in many applications. For example, ADC""s are used to convert an analog signal to a digital bit representation for analog video signal, audio signals and any other suitable analog signals. When converting analog video signals, or any other analog signals, the amount of precision (number of accurate bits), speed of operation, and cost are important factors in designing suitable analog to digital converter. Often, a 10 bit ADC may provide 8 bits of accurate information. Higher precision ADCs are often larger in die size and require changes to gains and offsets for the complex gate architecture. In addition, the more accurate the A/D converter, the longer the A/D converter takes in terms of clock cycles to generate an accurate result.
For example, one type of conventional A/D converter may use a successive approximation technique which effectively performs a binary search in a digital analog look up table and using a digital to analog converter (DAC) and comparator circuit, compares a sampled analog input signal with an output from the DAC which is a value from the lookup table. The difference between the signals is used to select a higher value or lower value from the lookup table. Such successive approximation techniques can be too slow for higher speed applications. For example, an eight bit A/D result may require 10 clock cycles to produce.
One form of successive approximation device includes tracking analog to digital converters, as known in the art. The tracking A/D converter is a type of successive approximation ADC. A tracking ADC may use a digital to analog converter in a similar manner as noted above, along with an up/down counter, controlled by the output of the comparator, to select an appropriate value from the lookup table. For example, the output of the comparator tracks the analog input signal based on a previous comparison with the analog input signal so that the input signal matches the digital to analog value in a feedback fashion. The up/down counter is sequentially incremented or decremented depending upon the direction of the difference between the analog input signal and the output of the digital to analog converter. Again, such tracking A/D converters typically require numerous clock cycles to provide a complete A/D result.
Other known conventional A/D configurations include flash analog to digital converters. Flash analog to digital converters provide parallel conversion by having simultaneous comparators comparing an input analog signal with multiple different reference levels. For example, for an 8 bit analog to digital converter, 255 comparators may be used with 255 different reference levels derived from a resistor ladder. This arrangement can provide an analog to digital result within one clock cycle. However, such flash analog to digital converters typically require an enormous number of gates. Consequently, such flash A/D converters can be susceptible to fabrication process variations, can typically consume large amounts of power and can be quite costly.
One solution has been to provide a folding analog to digital converter, such as those described, for example, in product literature entitled xe2x80x9cHigh Performance 8 bit Video Data Convertersxe2x80x9d, published by Philips Semiconductors, June, 1994, pp. 26-32, incorporated herein by reference. Another folding ADC arrangement is disclosed in an article entitled, xe2x80x9cAn 80 MHz, 80 mW, 8 b CMOS Folding A/D Converter With Distributed Track-and-Hold Processing,xe2x80x9d authored by Ardie G. W. Venes et al., published in the IEEE Journal of Solid-States Circuits, Vol. 31, No. 12, December 1996, pp. 1846-1853, incorporated herein by reference. Some folding ADC""s may be considered multistage A/D converters since they provide a coarse A/D result and also provide a fine A/D conversion in parallel. Such an arrangement can provide advantages by being smaller than flash A/D converters and faster than conventional successive approximation A/D converters. Such known multistage A/D converters typically provide A/D processing of coarse conversions and fine conversion in parallel. However, such converters can be very difficult to implement for high resolution output.
Accordingly, a need exists for an improved A/D converter that can provide relatively fast A/D conversion at a relatively low cost.